I dont really post in this thread anymore but I think I can give a partial answer to most images above.
Danbooru is an art repository that allows pornographic art but sexually explicit images are less likely to be approved than other images.
We have about 14 active approvers (some of which looks at all the images but doesnt click pass), any one of them can approve a post if they like it. Most of them have been on the site for years and has seen a lot of anime girls (more than three millions for the oldest). A simple drawing of a smiling girl is thus also less likely to be approved these days. Quality requirements has increased, if not intentionally.
Then we have a lot of Builders with unlimited upload privileges. They keep uploading art with the same quality as they always have and those posts dont go through the approval que. If your asking "how did this get approved but my image didnt", check if there is an approver listed. If not then It wasnt actually approved.
Its not a great system but its the best we have been able to come up with. Art is always subjective. Any artist can have a slump for a while or change their style to something none of our approvers like, so "This artist used to get approved" isnt really an argument.
Hope this helps some of you.